Choosing to buy a 'runt' can be a very loving gesture, but they must be raised carefully and you should be aware that the probability of health problems is greatly increased. Even the very best breeders, such as Kennel Club registered breeders in the UK, will occasionally find a runt in the litter. A responsible pug breeder will be careful to ensure that they are raised appropriately before it is time for them to be sold to their new home, and to ensure that the buyer knows what they are getting; they should be prepared for the possibility of extra health care costs over the life of the dog. There is nothing wrong with buying a runt, just so long as it is for the right reasons, and there is nothing wrong with a breeder selling a runt - just so long as they are not breeding weaker dogs intentionally and marketing them as 'miniature pugs'. Often a 'runt' will remain much smaller than dogs of a comparative age for their entire lives (and that is often a much shorter life), but sometimes they can also catch up with an infant growth spurt.
There are already a wide number of potential health problems in the Pug breed, even when buying from a top breeder, but the biggest problems come from the existence of interbreeding in a pugs family tree. Irresponsible breeders can be incompetent or irresponsible, often they are just greedy. An inbred group of pugs are much more likely to be born weak, and I would personally be very worried about the possibility of any 'miniature pug' breeder resorting to inbreeding. If they are irresponsible enough to select weak dogs for breeding, then they are probably irresponsible enough to inbreed. Ultimately though, if you want a tiny dog then get a chihuahua; or maybe even a 'chug' which is a chihuahua-pug cross. Or alternatively maybe a Boston Terrier? There are other pedigree dogs which grow to be smaller than a normal and healthy adult pug, do not resort to those that play with genetics.